1. Field of the Invention
The present invention relates to a color electrophotographic apparatus which performs halftoning operations through use of halftone spots formed by a plurality of dots, a method of processing an image produced by the color photographic apparatus, and a recording medium having recorded thereon a program to be used for image processing. In this specification, the word “dot” implies “pixel” defined as a unit cell in electrophotographic apparatuses.
The present application is based on Japanese Patent Application No. Hei. 11-28666, which is incorporated herein by reference.
2. Description of the Related Art
In an electrophotographic apparatus, such as a color printer or a color copier, a color image is reproduced by utilization of cyan toner, magenta toner, yellow toner, and black toner. Particularly, among color printers, some page printers—which forms a latent image on a photosensitive drum by utilization of a laser beam, develop the latent image by use of charged toner, and transfer an image formed from the thus developed toner onto transfer paper—can change an area to be exposed by the laser beam within a dot in various manners. Thus, even when the number of dots per unit area is small, those page printers can reproduce a color image with high resolution and high gradation.
In such a color electrophotographic apparatus, a dithering method has been widely utilized as a binary-coding method to be used for reproducing the halftone of a gray-scale image. According to the dithering method, by reference to conversion tables which are called dither matrices or threshold-value matrices and which define the correspondence between halftone data and image production data, a determination is made as to whether color spot is displayed in each dot or not. A dot is ‘ON’ when color spot is displayed and “OFF” when color spot is not displayed. Halftone spots are produced by one dot or some adjacent dots turning “ON”, and halftones of the images are reproduced on the basis of the sizes of halftone spots.
Dots are arranged in the direction of primary scanning in which a laser beam is moved for scanning (hereinafter referred to simply as a “primary-scanning direction”) and in the direction of secondary scanning in which transfer paper is fed (hereinafter referred to simply as a “secondary-scanning direction”). As some dots become “ON” and thus form the “core of the growth” of halftone spots. As the gray-scale level of the halftone data is increased further, the number of “ON” dots is eventually increased, thus gradually enlarging the size of the halftone spots.
FIG. 1 shows the combination of the angle of a cyan screen, the angle of a magenta screen, the angle of a yellow screen, and the angle of black screen, which has conventionally been used in wide applications of industrial printing systems. As shown in the drawing, according to the conventional technique, the angles of four color screens are set; specifically, the angle of the yellow (Y) screen is set to 0°; the angle of the cyan (C) screen [or the angle of the magenta (M) screen] is set to 15°; the angle of the black (K) screen is set to 45°; and the angle of the magenta (M) screen [or the angle of the cyan (C) screen] is set to 75°.
It is also known that, if the screen angles of the halftone spots are shifted in order to prevent chromatic misregistration, a so-called moiré pattern appears. It has empirically been acknowledged that a shift of angle of about 30° between two color screens is optimal for increasing the spatial frequency of the moiré pattern, to thereby render the moiré pattern inconspicuous. Yellow is less noticeable to the human eye. Therefore, the other 3 color screens (C,M,K) are set shifted from each other by 30°. Further, the angle of the black screen, which is most noticeable to the human eye, is set to 45°, so as be most distant from a longitudinal angle of 0° and a horizontal angle of 90°, which are easily recognized by the human eye. The angle of the cyan screen is set to 15°, and the angle of the magenta screen is set to 75°. The angle of the yellow screen is set to 0°. Although the yellow screen is set to the longitudinal direction or the horizontal direction that are most noticeable to the human eye, the yellow screen does not become greatly noticeable, because yellow is least noticeable to the human eye.
As mentioned above, the industrial printing system is designed so as to prevent a moiré pattern by setting the magenta or cyan screen to an angle of 15° or 75° and rotating the color screens. Since the color screens are only rotated, exactly as they are, the pitch among halftone spots is maintained uniform throughout the 4 colors.
In an electrophotographic apparatus utilizing a laser beam, the pattern of dots, which can be developed by an engine for developing an actual image on the basis of image reproduction data, is limited to the direction of primary scanning in which a laser beam is actuated for scanning, as well as to the direction of secondary scanning in which paper is fed. Unlike the industrial printing system, the electrophotographic apparatus is incapable of rotating the color screens to arbitrary angles. Accordingly, in the electrophotographic apparatus, desired screen angles are achieved by shifting the positions of the dither matrices to be used for the dithering method in primary or secondary scanning direction, or by changing the data in the conversion table, as required.
FIG. 2 is an illustration for describing a conventional method of determining screen angles in dithering method. In this example, dither matrices 40, each measuring m×m, are shifted from one another so as to correspond to image data, thus achieving a screen angle θ; i.e., tan θ=b/a. In a more specific example shown in FIG. 2, dither matrices 40 are shifted such that in a horizontal row of dither matrices 40, each dither matrix 40 is vertically shifted from the preceding dither matrix 40 by a given amount, such that after four shifts the last dither in the row is vertically shifted by an amount corresponding to the height of one dither matrix 40. Therefore, we have tan θ=¼. A dither matrix 42 designated by broken lines comprises a plurality of dither matrices 40. It is possible to determine the screen angle at an arbitrary value with higher degree of freedom by means of such a large dither matrix 42.
A screen angle of 15° for magenta and a screen angle of 75° for cyan, which are deemed to contributed to the best picture quality in the printing industry, are related to an irrational tangent (i.e., a tangent which is an irrational number). Angles related to the irrational tangent cannot be reproduced, so long as a limited number of dots arranged in both the direction of primary scanning and the direction of secondary scanning are utilized. For this reason, in the conventional electrophotographic apparatus, the magenta screen and the cyan screen are set to angles which are related to a rational tangent [tan θ=a/b (where “a” and “b” are integers), and θ=15° and θ=75° are not related to the rational tangent] and close to an angle of 15° and an angle of 75°.
Another conceivable approach toward selecting angles which are related to the rational tangent and close to 15° and 75° is to increase the size of the dither matrices 42. However, the number of dots per unit area which the engine can process is as small as, e.g., 600 dpi (dots per inch). If the size of the dither matrices is increased, halftone spot pitch increases and screen frequency is diminished. Further, an increase in the size of dither matrices also results in an increase in the number of corresponding required γ tables. Such an increase in the number of γ tables in turn involves an increase in the volume of a recording medium for recording the conversion tables Eventually, the cost of the electrophotographic apparatus is increased.
In a case where halftone spots are formed by utilization of the dots fixedly arranged in both the direction of primary scanning of the laser beam (i.e., the primary-scanning direction) and the direction of secondary scanning of the same (i.e., the secondary-scanning direction), the pitch between halftone spots among the color screens of different angles cannot be made uniform. Even in this respect, the electrophotographic apparatus encounters difficulty providing the same picture quality as that provided by the industrial printing system.